L_16_09 - 318N Krische, Lecture 16: Th - 03/12/09, EAS and...

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318N – Krische, Lecture 16: Th - 03/12/09, EAS and NAS In lecture, we considered para- versus meta-attack of electrophiles in the EAS reactions of toluene and anisole (methoxy benzene). We conclude that para-attack yields the more stable carbocationic intermediate because the resulting carbocation can be placed directly adjacent to an electron-stabilizing group. We conclude that electron- donating groups (EDG’s) are equivalent to cation stabilizing groups and, hence, EDG’s are ortho/para-directing groups. A further generalization: any substituent with a lone pair directly adjacent to the benzene ring will be an ortho/para-director. Electron withdrawing groups (EWG’s) will destabilize a carbocation. In the case of benzenes substituted with EWG’s, meta-attack is favored since the cationic intermediate derived from meta-attack does not place the formal positive charge directly adjacent to the EWG.
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L_16_09 - 318N Krische, Lecture 16: Th - 03/12/09, EAS and...

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